This invention relates to a thermal transfer recording medium and particularly to a hot-melt thermal transfer recording medium exhibiting high sensitivity and excellent performance in recording on an image-receiving element having a flat and smooth surface. The thermal transfer recording medium of the present invention is particularly useful for recording on a flat and smooth image-receiving element to produce a negative mask for image formation.
As is well known, hot-melt thermal transfer recording process is a recording system in which a thermal transfer recording medium comprising a substrate in sheet form coated thereon at least one hot melt ink layer is superposed on an image-receiving element with its hot melt ink layer contacting with the image-receiving element, and the hot melt ink layer is imagewise melted by the action of heat from the substrate side by means of a thermal printing head and transferred onto the image-receiving element to give a transferred image. This recording system has recently been widespread because of its utility in printing units featuring small size, lightness, competitive price and easy handling.
One of the characteristics of the hot-melt thermal transfer recording system is high freedom of choice in the kind of image-receiving elements to which it is applicable. That is, recording or printing can be conducted on not only common paper but fabric, plastic sheets or films, etc. In particular, with the recent spread of overhead projectors (OHP), occasion to record information on a transparent image-receiving element, such as a flat, smooth and transparent plastic sheet or film, has been increasing, and accordingly, the need for thermal transfer recording on such a transparent image-receiving element has been increasing.
However, when a conventional thermal transfer recording medium is used in printing on a flat, smooth and transparent image-receiving element, such as an OHP sheet, the resulting image suffers from blur, loss of definition, thickening of letters, or the tail of the printed image with reference to the recording direction and has disturbed or unprinted portions. These image defects are considered to be ascribed mainly to running or stringing of the ink at the time of ink transfer.
In order to overcome the above problem, it has been proposed to provide an ink-receiving layer on the surface of an image receiving element as disclosed in Japanese Patent Laid-Opens 61-135791 and 1-45688. However, the proposal is not feasible for practice because the ink-receiving layer not only incurs an increase of cost but reduces transparency of the image-receiving element, resulting in reduction in contrast between image areas and non-image areas (background).
On the other hand, it is known that a negative mask for image formation can be prepared by directly printing a negative image on a transparent material by making use of the hot-melt thermal transfer recording system. According to this method, no post treatment is required, and a mask can easily be obtained by means of general purpose type business machine adopting a thermal transfer recording system, such as a word processor or a printer.
Nevertheless, conventional thermal transfer recording media could not provide a negative image having a transmission density of at least 1.5 as is required for serving as a mask.
In order to solve the above problem, multiple printing or increase of the amount of the ink transferred has been attempted. In these cases, however, the resulting negative image has a reduced resolving power, and a mask with satisfactory image quality cannot be obtained.